Biden’s being paid for is part one. Part two is that no one in the State dept wanted to address it, thus leaving it to Trump to advance in a call.But the basis for impeachment wasn't that he investigated an innocent man. The basis for it was the "how," not the "what." The "how" matters.
Think about the reverse. Suppose Hunter Biden could somehow completely and conclusively prove his innocence. Would that make Trump's actions more impeachable? No, because that wasn't the issue.